1. Field of the Invention
The invention relates to a spectrum analyzer in which a frequency conversion takes place at at least two stages and which permits the frequencies of at least two such local signals to be changed and which also permits the frequency of a local signal of a later stage to be changed while holding the frequency sweep suspended for the local signal of an earlier stage.
2. Description of the Related Art
FIG. 1 shows an example of a general arrangement of a conventional spectrum analyzer of the kind described. An input signal is applied to an input terminal 14, and passes through an input variable attenuator 16 of a spectrum analyzer 15 to be fed to an earlier stage frequency converter 17 where it is mixed with a local signal from a frequency sweep oscillator 18. A resulting intermediate frequency signal is extracted by a bandpass filter 19, amplified by an amplifier 21 and then fed to a later stage frequency converter 22 where it is again mixed with a local signal from a local oscillator 23 to provide another intermediate frequency signal, which is then extracted by a bandpass filter 24 and detected by a detector 26. The detection output is passed through a low pass filter 27 and then converted into a digital signal by an A/D converter 28 to be stored in a buffer memory 29. A controller 31, which comprises a microprocessor including a CPU, sets up an attenuation in the attenuator 16 in accordance with a parameter to be measured which is entered by parameter presetting means 32, and also acts through a timing controller 33 to control a ramp voltage generator 34, thus establishing and controlling a sweep frequency band of the oscillator 18. The controller 31 also controls a display of data stored in the buffer memory 29 on a display 35.
As a consequence of this, the display 35 indicates how the status of frequency components in the input signal is, thus permitting an analysis in the frequency domain. When a desired frequency is entered, a frequency of the local signal which enables the extraction of the desired frequency component remains to be established by the ramp voltage generator 34, allowing the progress of the desired frequency component in the input signal with time to be displayed on the display 35 and thus permitting an analysis in the time domain.
In order to enhance the resolution of the frequency analysis, a ramp voltage is caused to be generated by a ramp voltage generator 39 through a timing controller 38 while the sweep of the frequency sweep generator 18 is held suspended, and the ramp voltage is used to sweep the oscillation frequency of the later stage frequency sweep generator 23. In the prior art, it has been impossible to perform an analysis in the frequency domain and an analysis in the time domain simultaneously. In particular, it has been impossible to detect a specific area in the time domain and to detect components in the frequency domain for that area.